Abstract

The early Proterozoic Menominee Group and equivalents of the southern Lake Superior region everywhere except in the Marquette area, Michigan, consist of quartzite gradationally overlain by iron-formation, with total thicknesses on the order of 0.5-1 km. In the Marquette area, the Siamo Slate of the Menominee Group is present conformably between the basal quartzite and overlying iron-formation, and the total stratigraphic thickness is on the order of 2 km. Based on variations in regional stratigraphic thicknesses and sedimentary facies analysis, it is suggested that the Menominee Group strata in the Marquette area were deposited in a dominantly deeper-water basinal environment, whereas elsewhere in the Lake Superior region sedimentation took place in platformal and broad, shallower-water basinal environments. The Marquette basin filled with material deposited by turbidity currents and pelitic sediments, whereas the platformal and broad-basinal strata accumulated in a wave-agitated offshore-shelf environment. Banded iron-formation occurs in both basinal and platformal environments, indicating that deposition of these chemical sediments was perhaps independent of local variations in sedimentary tectonic setting. The preferred regional tectonic setting evokes shelf and basin sedimentation on a rifted passive continental margin.